cold air damming
DESCRIPTION
Cold Air Damming. Cold Air Damming. What is Cold Air Damming?. Cold Air Damming. Cold Air Damming is when cold air is locked or “dammed” into a location, often a valley or side of a mountain chain. Causes for this phenomenon include topography and high pressure systems. - PowerPoint PPT PresentationTRANSCRIPT
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Cold
Air
Damming
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Cold Air DammingCold Air Damming
• What is Cold Air Damming?
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Cold Air DammingCold Air Damming
• Cold Air Damming is when cold air is locked or “dammed” into a location, often a valley or side of a mountain chain.
• Causes for this phenomenon include topography and high pressure systems.
• Cold Air Damming is also known as CAD.
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Cold Air DammingCold Air Damming
• How does Cold Air Damming occur?
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How Does CAD Occur?How Does CAD Occur?
• Cold air is stable and dense, so it is not always easily disrupted or transported.
• Warm air often rides up and over the cold air that is locked in at the lower levels.
• Hills and valleys work to keep cold air bottled up, like a dam bottles up a reservoir.
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How Does CAD Occur?How Does CAD Occur?
• Terrain alone is usually not enough to cause Cold Air Damming.
• A high pressure system in place or having an influence is a big factor.
• High pressure funnels in cooler air into sheltered areas, such as valleys or on sides of hills or mountains.
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Cold Air DammingCold Air Damminga rough sketch of a hypothetical Cold Air Damming situation
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CAD EvolutionCAD Evolution1.High
pressure crosses over into a sheltered area.
~Radiational Cooling
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CAD EvolutionCAD Evolution2. Cold air is
now in place across the region.
~Cold air is dense and stable, so it will tend to remain.
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CAD EvolutionCAD Evolution3. Cold air
remains, but warm air is moving in.
~Cold air has yet to be overcome by the warm air.
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CAD EvolutionCAD Evolution4. Cold air is
bottled up in the valley.
~Warmer air works in aloft, but not at the lower levels
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CAD EvolutionCAD Evolution5. Cold,
dense air remains.
~Warmer air takes over aloft and eventually works its’ way down to the pool.
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CAD GraphicallyCAD Graphically
• What does CAD look like closer up on a weather map?
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CAD GraphicallyCAD Graphically
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Forecast soundings for same timeForecast soundings for same time
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Importance of CADImportance of CAD
• Why is Cold Air Damming important?
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The CAD ImportanceThe CAD Importance
• Cold Air Damming can cause localized precipitation-type variances.
• A shallow cold layer would promote ZR.
• A deeper cold layer would promote IP.
• If the layer is deep enough and extends high enough, snow could even fall.
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P-Type ExampleP-Type Example
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Recent CAD EventRecent CAD Event
• 12 to 13 February (weak CAD event)
• Low pressure moves up the coast.
• With high pressure in place prior, cold air remains and hangs tough.
• Coastal front has trouble moving inland.
• Low pressure tracks SE of the area.
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Wednesday Morning 13 FebWednesday Morning 13 Feb
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Wednesday Morning 13 FebWednesday Morning 13 Feb
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Effects on CADEffects on CAD
• What might effect Cold Air Damming?
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Effects on CADEffects on CAD
• High pressure location and intensity.– If the high is in good alignment, cold air
damming can be maximized.
– If the high is strong, it can also work to drive in or at least keep in colder air.
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Effects on CADEffects on CAD
• Local topography and elevation.– For the northeast, CAD occurs most effectively
on the east side of hills or mountains.
– If the valley is deep or the mountains are tall, the depth of the cold pocket can be maximized.
– If the terrain has gaps or wedges, cold air can escape/warmer air can seep in.
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Effects on CADEffects on CAD
• Frontal boundaries.– If a warm front moves in, the CAD is likely to
diminish as warmer air is advected*.
– CT example, a coastal front establishes a boundary between the air masses.
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*Warm Air Advection**Warm Air Advection*
• Warm Air Advection aloft does not necessarily or immediately hurt CAD.
• In fact, CAD can be maximized when WAA occurs aloft and CAA is at the lower levels.
• This causes a capping inversion, which creates a stable environment.
• Also, cold air is more dense than warm air and WAA needs to mix and work in.
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Effects on CADEffects on CAD
• Mixing.– When warmer air moves in aloft and mixing
occurs, the cold wedge is disrupted.
– Colder air can be released.
– Warmer air can be filtered and mixed in.
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Effects on CADEffects on CAD
• Cloud cover.– Clouds can work to block insolation and
prevent the cold wedge from warming.
– Also, clouds and precipitation lead to evaporational cooling to enhance CAD.
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CAD DecayCAD Decay
• How might the cold wedge begin to diminish?
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CAD DecayCAD Decay• Wind flow change.
– If the low pressure system moves too close or to the northwest of the area, warmer air drives in. (warm sector)
– Wind flow effects how cold air is dammed into a location.
– Wind flow may also effect how warmer air can be driven in or colder air driven out.
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CAD DecayCAD Decay
• Mixing.– When warm and cold air interact, they mix and
the temperature changes.
– Mixing may be due to rising/sinking motion…
– turbulence created by surface roughness…
– strong winds…
– wind shear in the vertical profile.
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CAD DecayCAD Decay
• *Cold air advection*– If cold air advection occurs aloft, it can actually
work to decay the cold air damming.
– CAA favors sinking motion and tends to diminish cloud-cover and cloud development.
– This would work to allow solar heating to take place and to warm the layer.
– The capping inversion is disrupted.
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CAD in the NortheastCAD in the Northeast
• Discussion on cold air damming in the northeast and across the tri-state region.
What areas tend to get the best CAD?
What other local factors besides hills and valleys may affect CAD?
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CAD and NWPCAD and NWP
• Why do models have difficulty with CAD?
• What models would you expect to forecast CAD the most effectively?